Choosing the Right Potassium Fertilizer for Nut Orchards: Post-Harvest to Early Season

Fall fertility sets the foundation for next season’s nut yield

Fall isn’t the finish line—it’s the foundation. Especially in years when we face unpredictable weather, including rain and cold temperatures, during the crucial bloom period, setting the stage for a strong spring is critical post-harvest. In the fall, nut trees continue to take up nutrients through their roots, storing reserves that fuel bud development and next year’s crop. Even when trees appear dormant, they’re actively recovering from the nutrient drain of kernel, hull, and shell removal. Post-harvest activities, such as continuing irrigation, applying potassium (K), and addressing micronutrients like boron and zinc, all help ensure orchards enter winter strong and set the stage for consistent yields.

Why potassium matters

Almonds are among the hungriest crops for K. UC Davis research shows that approximately 80 lb. of K₂O are removed per 1,000 lb. of harvested kernels. Fruiting spurs and hulls are major “K sinks,” and failure to replenish losses can lead to spur death, reduced flowering, lower nut set, and reduced tree health. While Pistachios require about half the total K of almonds in their “on” years, K is critical for nutrient replenishment and sustained yields for all nut orchards.

Why sulfur is important for almonds after harvest (and what does it have to do with potassium?)

While potassium is the primary nutrient removed during almond harvest, sulfur (S) is critical for making that K work efficiently. Almonds remove large quantities of K, and S plays a key role in the plant’s metabolism, protein synthesis, and nutrient utilization. Maintaining adequate S levels alongside K is essential for tree health, growth, and yield.

Sulfur helps:

• Improve nitrogen use efficiency, supporting protein formation and tree vigor
• Activate enzymes involved in energy transfer and nutrient metabolism
• Support root activity and uptake during dormancy and early spring

Post-harvest, or dormant/early spring season S applications—especially in sulfate form—can help trees recover from the nutrient drain of harvest and prepare for strong bud development. This is why multi-nutrient sources like Polysulphate®, which deliver S alongside K, magnesium (Mg), and calcium (Ca), is increasingly favored by growers aiming for balanced fertility, flexible application windows and long-term soil health.

When is the best time to apply potassium?

While spring K availability is critical and early season applications help support yield outcomes, fall or dormant season fertility lays the groundwork for root uptake and bud strength before bloom. Fall or dormant season/early spring is an excellent time for soil-applied K. Applying after leaf drop allows winter rainfall to move nutrients into the root zone. Depending on logistics and precipitation, Polysulphate can be applied anytime from post-harvest through March with proven efficacy. This flexibility can be key for growers managing variable weather and labor schedules.

Where and how should potassium be applied in orchards?

Potassium can be soil-applied, injected via fertigation, or applied as foliar sprays. In drip-irrigated orchards, potassium, like Nova PeKacid®, Nova HiPeaK® or Nova FINISH™ can be run directly through the irrigation system. In our Polysulphate trials, we have found that side-discharge broadcast spreading along the tree line—banding the granular fertilizer along the tree rows where it will receive irrigation—offers excellent nutrient distribution and uptake. This method helps concentrate nutrients where roots are most active and irrigation is concentrated, while minimizing fixation and loss. Foliar K application choices are available with different analyses to meet orchards’ demands, such as Formula 1 0-29-26 (phosphite), Nova HiPeak 0-44-44, Nova FINISH 8-12-40, Nova FLOW™ 5-10-20, or Uptake K 0-0-23.

What are the best sources of potassium for post-harvest/dormant season/early spring applications on nut trees?

Choosing the right source post-harvest helps ensure consistent yields, nutrient balance, and long-term orchard resilience. Polysulphate delivers K, S, Ca, Mg in a low-salt, gradual-release form that supports orchard recovery and soil health. SOP remains a trusted standard, MOP is cost-effective but risky in sensitive soils, Langbeinite is helpful when magnesium is critical and Polysulphate stands out as the most complete and flexible option.

Comparison of Granular Potassium Fertilizer Sources for Almond Orchards (Fall Application)

Why are more growers choosing Polysulphate?

Polysulphate delivers four essential nutrients—K, S, Ca, Mg—in a single low-salt, pH-neutral fertilizer. While lower in K than traditional fall-applied inputs, the addition of the S and Mg help improve K uptake for a balanced approach. The gradual release of all four nutrients matches crop uptake, builds strong roots and increases NUE, even of N and P, although not part of the fertilizer. Research has shown increased nutrient use efficiency (NUE) with Polysulphate by 30-50%. Unlike other sources, it provides root/seed safety, gradual-release nutrition, S for improved nitrogen efficiency, plus Mg and Ca to support ion balance and stress resilience. It is OMRI-listed for organic use and enhances long-term soil health. In California almond trials, post-harvest / dormant season Polysulphate applications increased kernel yield by 4.8%, showing the payoff of investing in balanced fertility. Pistachio yield in California increased by 2149 lb./acre, plus 5% more split in shell, 32% reduction in blank nuts and 5% increase in dry weight with Polysulphate compared to gypsum.

Other post-harvest considerations:

• Irrigation: Continue watering after harvest to reduce stress and support bud development.
• Micronutrients: Apply boron and zinc, like the X-TRA micronutrient portfolio, via fall or early spring foliar sprays to replenish nutrients lost in hulls and support flower development.
• Water efficiency: Consider using H2Flo to improve water penetration and retention.

Bottom Line for Growers

Post-harvest/dormant/early spring season K management is one of the most critical steps for ensuring next season’s almond, pistachio, walnut, and pecan yields. While SOP remains a reliable standard, MOP carries chloride risks, and Langbeinite is situational—Polysulphate® stands out as one of the safest, most flexible, and most comprehensive solutions. By replenishing K alongside S, Mg, and Ca, Polysulphate® supports orchard recovery, soil health, and yield and quality stability—all while offering flexibility of OMRI-approved application from harvest through early spring.

Sources:

• Farm Progress. (2020). As almond harvests end, postharvest chores begin. https://www.farmprogress.com/tree-nuts/as-almond-harvests-end-postharvest-chores-begin

• Geisseler, D. (n.d.). Almonds. UC Davis Nutrient Management Guidelines. http://geisseler.ucdavis.edu/Guidelines/Almonds.html

• Reidel, E. J., Brown, P. H., Duncan, R. A., & Weinbaum, S. A. (n.d.). Almond productivity as related to tissue potassium.

• Sacramento Valley Orchards. (n.d.-a). Postharvest nutrition review. https://www.sacvalleyorchards.com/almonds/horticulture/postharvest-nutrition-review/

• Sacramento Valley Orchards. (n.d.-b). Potassium management for sustained almond yields. https://www.sacvalleyorchards.com/almonds/horticulture/potassium-management-for-sustained-almond-yields/

• The Almond Doctor. (2015). Late fall: A time for potassium soil applications. https://www.thealmonddoctor.com/blog/late-fall-a-time-for-potassium-soil-applications

• UC Agriculture & Natural Resources. (2011). Fertilizing almonds in California (Publication 3364). https://ucanr.edu/sites/default/files/2011-06/111428.pdf